def p (n):
    for i in range(2,n):
        if n % i == 0:
            return False
    return True

x = int(input())

a = [i for i in range(2, x+1) if p (i)]

for i in range(2, x+1):
    if p (i):
        a.append(i)
m = 0
for i in a:
    if str(i) in str(x):
        m=i
print(m)
